This invention relates to a closed-cell rigid polyurethane foam having a remarkably low density as well as a low thermal conductivity and also well balanced in its various properties, a process for producing such polyurethane foam, and use thereof for heat insulators and refrigerators.
Rigid polyurethane foams are usually obtained by reacting a polyol component and an isocyanate component in the presence of a blowing agent, a reaction catalyst and a foam stabilizer. Since closed-cell rigid polyurethane foams are generally excellent in heat insulating properties, they are widely used as various kinds of heat insulating materials. Request is strong, however, for those polyurethane foams which are lower in density, smaller in thermal conductivity and better in moldability than those hitherto available. Various proposals have been made to improve these matters.
Blowing agent exerts a great influence on formation of closed cells and properties of foams and should be given a serious consideration in its use and treatment. Among the blowing agents generally used, chlorofluorocarbon (CFC), which is hardly decomposable, is attracting attention as its accumulation in the atmosphere could lead to destruction of the ozone layer and world-wide discussions are being made for restricting use or even production of this compound. Under such circumstances, measures against use and treatment of this compound are urgent.
It has long been known in the art that water is usable as a blowing agent. For example, Japanese Patent Application Kokai (Laid-Open) No. 53-9797 proposes use of a fluorochlorinated hydrocarbon and water (0.2 to 3 parts by weight per 100 parts by weight of polyol component) as a blowing agent. It is stated that, in this case, the amount of water usable for the composition is subject to certain limitations since use of an excess amount of water not only leads to consumption of polyfunctional isocyanate component to cause a poor economy but also notably reduces flexibility of the produced foam while also deteriorating the heat insulating properties. Use of water, however, had a serious problem in that its use even in a small amount could cause deterioration of heat insulating properties or formation of gas boils in the product, resulting in unbalance of the properties required for a rigid urethane foam.
For maintaining a good balance of properties, use of a combination of specific polyol components is proposed in Japanese Patent Application Kokai (Laid-Open) No. 59-84913, but as regards the blowing agent, it is simply mentioned that trichloromethane and water are usable, with no reference being made to the amount of water to be used. Further, in Japanese Patent Application Kokai No. 62-81414, it is stated that water used for the blowing agent exerts no influence on thermal conductivity if the amount of water used is less than 1% by weight in trichlorofluoromethane, and it is rather helpful for maintaining appropriate fluidity even when the mold temperature is low. This proposal is no more than a reconfirmation of the old concept that the desired properties cannot be obtained unless the amount of water used is as small as less than 1% by weight in the blowing agent. This reflects the present state of technology in this field of art.